The present invention relates to new solvents for use with forward osmosis processes. Desalination through reverse osmosis is a known technique in the field of water treatment. Generally, reverse osmosis desalination involves artificially adding a relatively high pressure to move water in the opposite direction through a membrane, thereby producing fresh water. Since reverse osmosis requires a relatively high pressure, it also has high energy consumption, typically around 2.0-3.5 kWh/m3 of fresh water produced. Thermal desalination processes are also well-known techniques for both seawater and brackish water treatment, with a typical energy consumption in the range of 14-18 kWh/m3 of fresh water produced, unless free steam is available on site.
Forward osmosis (FO) is a process technology being explored for desalination of seawater, as well as treatment of industrial waste water and other saline waters. Unlike reverse osmosis (RO) processes, which employ high pressures ranging from 400-1100 psi to drive fresh water through the membrane, forward osmosis uses the natural osmotic pressures of salt solutions to effect fresh water separation. A draw solution having a significantly higher osmotic pressure than the saline feed-water flows along the permeate side of the membrane, and water naturally transports itself across the membrane by osmosis. Osmotic driving forces in FO can be significantly greater than hydraulic driving forces in RO, leading to higher water flux rates and recoveries. Thus, it is a non-pressurized system, allowing design with lighter, compact, less expensive materials and low-pressure pumps. These factors translate in savings both in capital and operational costs. Energy represents about 40% of the costs of RO desalination, (and around 80% of the costs of thermal desalination). In addition, the lower amount of more highly concentrated by-product brine is also more easily managed.
As a solute for the osmosis draw solution, ammonium bicarbonate, sulfur dioxide, aliphatic alcohols, aluminum sulfate, glucose, fructose, potassium nitrate, and the like have been used. Among them, an ammonium bicarbonate draw solution is most commonly known, which may be decomposed into ammonia and carbon dioxide and separated at a temperature of about 60° C. after forward osmosis. Furthermore, newly suggested draw solution materials include thermosensitive polymers, magnetic nanoparticles having a hydrophilic peptide attached thereto (separated by a magnetic field), a polymer electrolyte such as a dendrimer (separated by a UF or NF membrane), and the like.
In the case of ammonium bicarbonate, the diluted draw solution needs to be heated to about 60° C. or more so as to vaporize the ammonium carbonate, thus requiring higher energy consumption. Also, since complete removal of ammonia is practically difficult, it is less than desirable to use it as drinking water due to the odor of ammonia. In the case of using magnetic nanoparticles as the active component of the draw solution, it is relatively difficult to redisperse magnetic particles that are separated and agglomerated by a magnetic field. It is also relatively difficult to completely remove the nanoparticles, and thus the toxicity of the nanoparticles should be considered. In the case of a draw solution made from a polymer electrolyte, polymer ion (dendrimer, protein, etc.) technology requires a nanofiltration or ultrafiltration membrane filter due to the RH size of the polymer of several to dozens of tens of nanometers. It is also relatively difficult to redisperse the agglomerated polymer after filtering.
Thermosensitive polymeric solutions have been considered as suitable osmotic draw solutes. These polymers have a tendency for phase separation from their water solutions at a critical temperature, and thus can be suitably separated from the permeated water of the FO process. Both lower and upper critical temperatures have been exhibited, depending on the configuration of the polymer molecule. At the lower critical temperature, the polymer separates into a hydrophobic layer from the water, and thus, can be re-concentrated by nanofiltration or other techniques for recycling as a concentrated draw solute for the next cycle of FO. Some polymers can re-dissolve in water above the upper critical temperature.
Polyethylene glycols (PEGs), polymers of ethylene glycol (EG), have been used in industry to produce very high osmotic pressures, in the order of tens of atmospheres. In comparison, seawater (3.5% NaCl) has an osmotic pressure of 28 atms at 25° C. PEGs are hypotonic by nature, and absorb water exceedingly well. The hydrogen bonding between water molecules and the electron-rich ether oxygen in the EO (ethylene oxide) monomer enables almost 2.5-3.0 molecules of water to be coordinated with each EO monomer, leading to high osmotic pressures. Thus, the greater the number of EO monomers in the PEG molecule, the greater the osmotic pressure exhibited. One issue with longer chain-length PEGs is the higher viscosity and higher melting points, as the chain length increases. PEG 200 (EO=4), PEG 300 (EO=6-7) and PEG 400 (EO=9) are all liquid at room temperatures, whereas PEG 600 (EO=12-13) is a waxy solid at room temperature, as are the higher molecular weight PEGs. Thus, a practical limit in the PEG chain length prevents use of longer chain-length PEGs for water absorption.
If an hydrophobic entity, like propanediols or butanediols, is attached to the PEG molecule, the hydrophobic-lipophilic balance (HLB) of the copolymer can be suitably shifted, such that phase separation can occur at certain temperatures, usually termed cloud-point or critical temperatures, as mentioned in the paragraph above. The draw solute copolymers consist of various numbers and orders of diols, which impart the required solution properties. Osmotic pressure, cloud point temperature, molecular weight and molecular structure are adjusted by adding or subtracting the various monomer units. Within the constraints of osmotic pressure and cloud point temperature, the chemistry of the draw solute polymers can be selected to control the molecular weight and/or physical structure of the polymer resulting in high (>90% and preferably >99%) rejection of the draw solute through filtration. Further, the chemistry of the draw solute polymers can be selected to minimize back diffusion of the solute through the forward osmosis membrane. Preferably, for salt water desalination, the osmotic pressure of a draw solution containing 40% draw solute copolymer in water needs to be greater than 30 atm, preferably greater than 40 atm, and more preferably greater than 50 atm for high water recovery from the saline feed solution, while the molecular weight of the draw solute copolymer is greater than 500, preferably greater than 1000 and more preferably greater than 2000.
While the PEGs used in these copolymers are linear in structure, and increase in melting point and viscosity as the chain-length increases, there are other forms of PEGs available, with different geometries, that are termed branched or multi-armed PEGs. Branched PEGs have 3-10 PEG chains emanating from a central core group. Star PEGS have 10 to 100 PEG chains emanating from a central core group, while comb PEGs have multiple PEG chains grafted onto a polymer backbone. Such branched PEGs allow more EO groups in the polymer, while remaining in the liquid state and having lower melting points and viscosity than comparable linear PEGs with the same number of EO monomers. Thus, the use of such PEG geometries can enable higher water absorption, while retaining the practicality of using higher number of EO monomers for water molecule interaction by hydrogen bonding.